Astringency is not a flavor (thank you, wine physiology)

It’s generally agreed that we have five basic tastes — sweet, salty, bitter, acidic, and umami — all of which make appearances in wine.** The nuances described in baroque tasting notes — fruits and flowers and tar and tobacco and the rest — are, of course, smells. But where does that leave astringency? In the hands of physiology researchers, evidently. Anatomy is the science of labelling the parts of the body and where all the bits are. Physiology is the science of understanding how those parts work. So when we ask questions about how wine triggers responses in the mouth, we ask physiology.

Astringency is the dry, rough, puckery feeling left in your mouth by a sturdy red wine, strong black tea, dark chocolate, or (best example ever) an underripe persimmon. Some astringent molecules also taste bitter, but that’s not what we’re talking about. Astringency doesn’t seem to be a taste. It’s definitely not a smell. It’s…something else. But since descriptions like “something else” leave scientists (and wine drinkers, maybe) feeling unsatisfied, physiology researchers at Ruhr University in Germany have been trying to pin down astringency more precisely.

Research on astringency isn’t new, but it’s been confusing. Astringency triggers the same nerve that’s used to carry flavor sensations in mice; since flavors and feelings (like touch and temperature) are carried by different nerves in the mouth, that’s a useful observation. But astringency can be sensed by parts of the mouth that don’t have taste receptors. We know (or we think we know) that tannins are responsible for red wine astringency, and what we’re taught in food science classes is that tannins bind to the proteins in your saliva and cause them to glom together, which simultaneously decreases the slipperiness of your saliva — making your mouth feel dry — and creates a bunch of big rough tannin-protein blobs that themselves feel rough. The problem with that explanation is that the intensity with which we sense astringency doesn’t seem to be related to how much protein gets bound up, and not all molecules that seem to cause astringency bind up proteins at all.

This new German study, unfortunately, doesn’t help resolve most of those conundrums. But it did use a simple, elegant little trick to pretty firmly say that astringency isn’t a taste, and that it is a feeling.

Since completely different nerves carry taste sensations and mechanical feelings like pressure or roughness back to the brain, these researchers used anaesthetic — the same injectable kind you’d get at the dentist — to numb up either the taste nerve for the front of the tongue alone or both the taste and the feeling nerve of some real live humans, then subjected them to astringent things like quinine (the stuff in tonic water) or powdered chestnut. They also found some folks whose mechanical feeling nerve had been cut in the course of middle ear surgery, which means that they couldn’t taste on half of the front of their tongues (these and most nerves are paired with one for the right and one for the left side of the body). The folks who couldn’t taste — either because of surgery or because of anaesthetic — had no trouble detecting astringency. But the folks who couldn’t feel were numb to the astringent sensations. That suggests that we don’t sense astringency in the same way as mice, but that’s not outside the realm of possibility.

The study also included tests on isolated nerve cells (from mice, not those human subjects; don’t worry) to look for exactly what molecules were triggering the mechanical nerves and what kind of triggering was going on. Those experiments showed that astringency isn’t just the roughness you feel when you move your tongue around, but that nerve cells are being activated directly. In other words, you could still feel astringency if you couldn’t tell whether the inside of your mouth was rough by moving your tongue and cheeks around.

But, still, that’s an open question: is the sensation of astringency caused just by chemicals triggering nerves, or is it also the product of rough feelings when we move our mouths around? Since knocking out the mechanical sensation nerves knocks out both of those feelings, these experiments couldn’t say.

So when astringency comes up over a glass of tannic red, you can continue to confidently say that the wine feels astringent rather than tasting astringent. Cocktail-party trivia, sure. And maybe this research has other functions, in understanding and knowing how to fix peculiar diseases of messed-up mouth nerves. But in some ways, it’s about what science has always been about: looking at something — in this case, our own bodies — and asking, “Well gosh, how does that work?” The earth is a giant puzzle book, and we’re certainly in no danger of reaching the last page any time soon.

**Yes, yes: what constitutes a basic taste is a matter of debate, but that’s an interesting topic for another day.